The invention pertains to a vacuum coating apparatus with a crucible in a vacuum chamber for holding the material to be evaporated, such as a metal or metal oxide or mixture of the two, and with a strip of film guided by rolls a certain distance away from the material to be evaporated.
U.S. Pat. No. 5,302,208 discloses a vacuum coating apparatus with a container for material to be evaporated, with an evaporation device for evaporating the material in the container, the substrate to be coated being a certain distance away from the material to be evaporated. A microwave transmitter transmits microwaves into the space between the material to be evaporated and the substrate to be coated. This known apparatus makes it possible to improve the properties of a metal oxide coating on a plastic film.
U.S. Pat. No. 4,956,070 discloses an apparatus for depositing a thin film by sputtering, in which the rates at which layers of different materials are deposited can be controlled, so that extremely thin layer packages can be produced. For this purpose, at least two different types of counterelectrodes are provided on the cathode side.
DE 35 41 621 discloses an apparatus for depositing a metal alloy by means of HF sputtering, in which two targets are driven in alternation. Both targets contain the metal components of the metal alloy to be deposited but in different proportions. For the purpose of deposition, the substrates are mounted on a substrate carrier, which is rotated by a drive unit during the sputtering process.
DE 38 02 852 discloses an apparatus for coating a substrate, including two electrodes and at least one material to be sputtered, in which the substrate to be coated can be set up between and a certain distance away from the two electrodes, and the alternating current half-waves can be selected as low-frequency half-waves with essentially the same amplitudes.
U.S. Pat. No. 5,415,757 discloses an apparatus for coating a substrate especially with nonconductive layers from electrically conductive targets in a reactive atmosphere. A current source is connected to cathodes which enclose magnets and which are installed in an evacuatable coating chamber, the cathodes working together electrically with the targets. Two anodes separated electrically from each other and from the sputtering chamber are installed in a plane between the cathodes and the substrate. Each of the two outputs of the secondary winding of a transformer connected to a medium-frequency generator is connected to a cathode by way of a power feed line, the first and the second feed lines being connected to each other by a branch line, into which an oscillatory circuit, preferably a coil and a capacitor, are inserted. Each of the two feed lines is connected both to the coating chamber by way of a first electrical element which adjusts the d.c. potential with respect to ground and also to the respective anode by way of a corresponding second electrical element as well as to the coating chamber by way of a branch line containing a capacitor. A choke coil is inserted in the segment of the first feed line which connects the oscillatory circuit to the second secondary winding.
Finally, DE 44 12 906 discloses a method for ion-assisted vacuum coating, preferably for the high-speed coating of large, electrically conductive or electrically insulating substrates with electrically insulating layers and for coating electrically insulating substrates with electrically conductive layers. In this method, a plasma is generated between a coating source and the substrate, and ions from the plasma are accelerated toward the substrate. Voltage pulses alternating between negative and positive relative to the plasma, are applied to the electrically conductive substrate or to an electrode mounted directly behind the electrically insulating substrate and extending over the entire surface to be coated. The duration of the negative pulses is adjusted to the charging time of the capacitor being formed over the insulating layer and/or the insulating substrate, whereas the duration of the positive pulses is at most equal to, preferably 2-10 times smaller than, the duration of the negative pulses. The positive and negative pulses succeed each other without pause and are adjusted approximately to the same amplitude relative to the base potential. The amplitude of the pulses relative to the base potential is adjusted to .+-.20 to .+-.2,000 V, preferably to .+-.50 to .+-.500 V.
Transparent plastic films are being used to an increasing extent for the packaging of food products. The films primarily in question here are made of polymeric plastics, which, although they are flexible, suffer from the disadvantage that they are permeable to aromatic substances, water, and oxygen. When it is desired to prevent the diffusion of such substances, the general practice is to use aluminum foils or plastic films onto which a layer of aluminum has been deposited from the vapor phase. These have the disadvantage, however, that they are relatively difficult to dispose of and are not transparent to microwaves or light. Because of the widespread use of microwave ovens in many households of industrialized countries, it is often important for a packaging material to be transparent to microwaves.
To combine the advantages of plastic films, which are transparent to microwaves, with the advantages of aluminum foils, which form an impenetrable barrier to aromatic substances, water, and oxygen, it is already known that polymer films can be coated with oxides. Silicon oxide plays the most important role as a coating material for this purpose. The properties of plastic films coated with silicon oxide are similar to those of aluminum foil or aluminum-coated plastic film with respect to the structure of the laminate and the barrier behavior with respect to oxygen, water vapor, and aromas.
The coating of plastic films with metal oxides such as SiO.sub.x, however, requires a process technology which differs sharply from the conventional coating technologies, because metal oxides, in contrast to metals, must be evaporated from the solid phase.
SiO.sub.x layers are produced by evaporating SiO by the use of an evaporation furnace or by means of an electron beam (see T. Krug and K. Rubsam: Die neue "glaserne Lebensmittelverpackung" in Neue Verpackung, Huthig-Verlag, 1991). SiO.sub.x layers offer the advantage that they are more flexible. In addition, SiO.sub.x is also chemically inert and corrosion-resistant with respect to water. Because it has a relatively high vapor pressure, SiO can, like MgO, Al.sub.2 O.sub.3, and SiO.sub.2, be evaporated by means of an electron beam.